Molecular recognition of ingested nutrients results in the secretion of gastrointestinal hormones regulating digestion and gut motility necessary for absorption of these nutrients. Amino acids, short chain fatty acids, carbohydrates and changes in intraluminal pH are known potent stimulators of gastrin, cholecystokinin and secretin, some of the most important gastrointestinal hormones regulating digestion, secretion and motility. The cells responsible for chemosensation of these nutrients are presumed to be scattered along the mucosal layer lining the gastrointestinal tract and may possibly be neurons within the enteric nervous system (3). The inherent dispersed nature of these chemosensory cells and the lack of any known markers for their identification create a difficult problem for solving the molecular basis of nutrient recognition. Calcium acting at the calcium sensing receptor on gastric G cells has been shown to stimulate the secretion of gastrin. During the past year, we have bred a sufficient colony of calcium sensing receptor null mice. In vivo, antral tissue and enriched gastrin cell preparations are now being used to investigate the role of the calcium receptor in the gastrin response to luminal meal components such as peptone, amino acids and calcium.